1. Field of the Invention
The present invention relates to a method and an apparatus for adjusting a density detecting device, which device is used for an image forming apparatus for forming an image by an electrophotographic process, for example, an electrostatic copying machine, and which device is for outputting density data utilized in adjusting the image forming conditions such as the amount of charge, the amount of exposure and the developing bias so as to keep the formed image high in quality.
2. Description of the Related Art
In the electrostatic copying machine, a copy image is formed in the following manner. Specifically, a real original which is put on a transparent platen to reproduce the image thereof is illuminated and scanned. Reflected light from the real original is introduced into a photosensitive drum which is rotated in synchronization with the illumination and scanning. As a result, the photosensitive drum is exposed. The surface of the photosensitive drum before the exposure is uniformly charged by a charger. An electrostatic latent image corresponding to the real original is formed on the surface of the photosensitive drum by selective charge elimination caused by the exposure.
The formed electrostatic latent image is developed into a toner image by a developing device to which toner is supplied from a toner hopper. The toner image is transferred onto copy paper by corona discharges in a transferring corona discharger. The copy paper on which the toner image has been transferred is introduced into a fixing device, where the toner is fixed to the copy paper, thereby completing copying.
An attempt to stably obtain an image high in quality in the above described electrostatic copying machine brings about the necessity of suitably adjusting the image forming conditions such as the amount of exposure and the amount of charge of the photosensitive drum, the developing bias and the amount of toner to be supplied to the developing device.
The image forming conditions are adjusted for each predetermined period, for example, at the time of maintenance. In adjusting the image forming conditions, a pure white or solid black pseudo original (a reference density original) which is arranged in a region other than a region where the real original is illuminated and scanned is experimentally illuminated, and a toner image corresponding to the pseudo original is formed. At this time, the amount of exposure, the surface potential, the density of the toner image on the surface of the photosensitive drum, and the like are detected, and the image forming conditions are automatically adjusted on the basis of the results of the detection. Specifically, where the pure white pseudo original is illuminated to form a toner image, if so-called fog is detected on the basis of the detected toner image density, the amount of exposure is increased. On the other hand, where the solid black pseudo original is illuminated to form a toner image, if it is judged that the density is insufficient on the basis of the results of the detection of the toner image density, toner is automatically supplied to the developing device from the toner hopper.
A reflection type photosensor which is constituted by a pair of a light emitting element and a light receiving element arranged opposed to the photosensitive drum is generally applied to the detection of the density of the toner image on the surface of the photosensitive drum. Specifically, light of a previously set amount is irradiated onto the photosensitive drum from the light emitting element, and density data corresponding to the amount of light reflected from the photosensitive drum is outputted from the light receiving element. Since the amount of the reflected light corresponds to the density of the toner image on the surface of the photosensitive drum, it is possible to detect the density of the toner image on the surface of the photosensitive drum on the basis of the above described density data.
At the time of initialization immediately after manufacturing the copying machine, two types of amounts of light to be irradiated, for example, an amount of light for low density and an amount of light for high density are set as amounts of light to be irradiated onto the photosensitive drum from the light emitting element in the reflection type photosensor. The amount of light for low density is the one to be irradiated onto the photosensitive drum from the light emitting element when a fog detection is performed. On the other hand, the amount light to be irradiated for high density is the one to be irradiated when a solid black detection is performed.
The reason why the amount of light to be irradiated is varied depending on a case where fog is detected and a case where a solid black is detected is as follows.
Specifically, when the fog detection is performed, toner hardly adheres to the photosensitive drum because the pseudo original on which a pure white image is formed is illuminated. Consequently, the amount of light received by the light receiving element is relatively high. On the other hand, an output of the light receiving element is saturated if the amount of received light is increased. Therefore, the amount of light to be irradiated when in the fog detection must be relatively decreased so as to restrain the amount of light reflected from the photosensitive drum.
On the other hand, when a solid black detection is performed, a large amount of toner adheres to the photosensitive drum because the pseudo original on which a solid black image is formed is illuminated. Consequently, most of light irradiated from the light emitting element is absorbed by the toner on the surface of the photosensitive drum, whereby the amount of light received by the light receiving element is relatively small. On the other hand, the light receiving element cannot detect a subtle change in the amount of received light if the amount of received light is small. Therefore, the amount of light to be irradiated in the solid black detection must be made relatively large so as to increase the amount of reflected light.
An amount of light for low density to be irradiated in the fog detection is found in the following manner by maximizing the amount of exposure to bring the photosensitive drum into an undeveloped state where no toner adheres on the surface.
Specifically, the photosensitive drum which has not been developed is rotated once, and light of a certain amount is irradiated onto the photosensitive drum from the light emitting element a plurality of times during the rotation thereby to find the average of density data corresponding to the amounts of reflected light outputted from the light receiving element. The photosensitive drum is rotated once in order to restrain the variation caused by irregularities in the circumferential direction on the surface of the photosensitive drum.
The density data are acquired with respect to a plurality of amounts of light to be irradiated in predetermined steps (for example, 100 steps). An amount of light corresponding to the density data selected on a predetermined basis out of the acquired density data corresponding to the amounts of light in the steps is set as an amount of light for low density.
On the other hand, an amount of light for high density to be irradiated in the solid black detection is found by substituting the obtained amount of light for low density in a predetermined conversion equation.
In the above described prior art, however, the density data corresponding to the amounts of light to be irradiated in the plurality of steps are acquired by rotating the photosensitive drum once for each of the amounts of light to be irradiated in the steps. Therefore, a long time is inevitably required to acquire the density data. Moreover, light is irradiated onto the photosensitive drum over a long period of time, thereby causing the early light-induced fatigue of the photosensitive drum.
On the other hand, if the number of steps of the amounts of light to be irradiated from the light emitting element in the reflection type photosensor is increased, the density of the toner image on the surface of the photosensitive drum can be detected with high precision, thereby making it possible to set the amount of light for low density to a very suitable value. If the number of steps of the amounts of light to be irradiated is increased, however, it takes much time to acquire the density data. Therefore, it is impossible to easily increase the number of steps of the amounts of light to be irradiated, and it is not altogether easy to set a suitable amount of light for low density with high precision.